KATAKANA is used when BORROWING words from the ENGLISH language or for NAMES
e.g. ERIKA エリカ
HIRIGANA is used for normal Japanese words and context (unless KANJI has been introduced)
e.g. GANBATTE がんばって

REGARDING THE "little tsu" [っ]
- is used when DOUBLE consonant...as shown in the word above.

wouldn't it make sense to separate the hirigana and katagana into two serparate line up(graph/chart/etc.) for beginners, cause they might be confused.
Also I dunno if you(I'm lazy to do this) want to explain about particles like で, は, を, と, の and some other ones